This invention relates to a method for the electrodeposition of metals, and, more particularly, to a method for continuously measuring and adjusting the concentration of very low levels of impurities, particularly antimony impurities, in electrolyte solutions, particularly zinc electrolyte, during electrolytic metal recovery operations.
In processes involving the electrodeposition of metals such as electrowinning, electrorefining and electroplating, electrolytic solutions are used which contain impurites that when present above certain predetermined concentrations can electrodeposit with the plated metal and thereby either contaminate or cause resolution of the deposit, with a corresponding decrease in the efficiency of the metal deposition process. Consequently, an effective technique for monitoring the purity of the electrolyte supplied to and used in the electrolysis section of the metal plant is required since the current efficiency is greatly affected by the presence of even minute quantities of undesirable impurity metal ions.
In modern industrial operations, metals such as zinc are produced from ores and/or concentrates by either roasting and leaching, or direct leaching so as to generate a solution, e.g., zinc sulfate, which contains a substantial amount and number of impurities. The solution is then purified by conventional techniques, thereby lowering the deleterious impurities to levels which will not interfere with the zinc recovery process. More particularly, when impurities such as antimony, arsenic, cadmium, cobalt, copper, germanium, nickel, selenium and tellurium are present above certain concentrations in various electrolyte solutions, ranging from 0.01 ppm (parts per million) for antimony to 2.0 ppm for cadmium in the case of zinc electrolyte, the hydrogen overpotential on zinc can be reduced, thus significantly impairing current efficiency while also permitting the dissolving of already deposited pure zinc metal.
The art has made many attempts to solve this troublesome problem, the most common being the use of analytical techniques for determining the concentration of the various impurities. However, such processes have all proven unsatisfactory due to the lengthy and unreliable techniques available for measuring the low levels of impurities found in the electrolyte. Additionally, these technique are deficient for a process control method since the variables to be monitored can frequently change significantly between the time the measurement is taken and the time the sample is quantitatively analyzed. The addition of organic reagents such as animal glue can inhibit the deleterious effects, but are frequently effective only within certain relatively narrow impurity ranges.
The importance of determining the quality of zinc electrolyte is reflected in the number of published references discussing the problem, and the number of different techniques investigated, none of which have gained wide acceptance in the zinc industry. One such technique, reported by R. C. Kerby et all. "The Construction and Operation of a Meter for Measuring the Quality of Zinc Electrolytes", Technical Bulletin TB 160, Dept. of Canadian Energy, Mines and Resources, September 1972, measures the evolution of hydrogen gas in a small cell as a measure of current efficiency. However, this only provides a qualitative indirect measurement and can be influenced by factors other than impurity levels. In U.S. Pat. Nos. 4,324,621 and 4,217,189, R. C. Kerby discusses many attempts to measure and/or determine the effects of impurities and discloses methods for measuring the activation overpotential between the cathode and a reference cell, and relates this to the concentration of impurities and polarizing affecting agents present in the sample. The processes for the purification of electrolyte and the electrodeposition of metals are subsequently adjusted in relation to the earlier measured value. A method for measuring the amount of current required to plate and then deplate, which can be related to the electrolyte quality, is disclosed by A. D'Este et al. in "Montevecchio" 16, Nos. 3-4, 1-11 (1965). A report by R. V. Wong of EG&G Princeton Applied Research entitled "Electrochemical Techniques for the Analysis of Plating Baths" discusses the use of differential pulse polarography to analyze major and minor constituents in a plating bath.
It is an object of this invention to provide a new and improved method for the electrodeposition of metals.
Another object is to provide a method for analyzing low levels of impurities, e.g., in electroplating solutions, by using a polarograph.
Other objects will be apparent from the following description.